According to the EN61000-3-2 IEC standard, there are imposed limits for harmonic current emissions (equipment input currents 16 A per phase). In one common application, active power factor correction (PFC) is used to convert the quasi sinusoidal input current from the line (main power supply) to a more sinusoidal form. This increases the power factor of the supply and thus reduces the harmonic current emissions, allowing the supply to meet EN61000-3-2 for Class A equipment.
FIG. 1 depicts a prior art power supply circuit 10 implementing active power factor correction. As shown in FIG. 1, the AC line voltage 12 is full-wave rectified by rectifier circuit bridge 15 and a switching regulator 18 including switching components Q1 and D1 that are used to boost up this voltage, e.g., to 380V, regardless of the input. Thus, universal input is achieved and the input current is forced to be sinusoidal and in phase with the line voltage by U1 controller 20. This control chip may comprise a typical average current mode controller, e.g., such as provided by Texas Instruments and defined as part number UC3853, or UC 3854. As shown in FIG. 2, the active power factor correction according to the circuit configuration of FIG. 1 enables maximum power transfer as the alternating line current 25 (I line) and the a.c. line voltage 29 (Vac) are in phase. The boosted 380V may then be further switched to provide a regulated DC voltage, such as 5V, 3.3V, etc., employed for semiconductor integrated circuits. The circuitry depicted in FIG. 1, although effective in reducing harmonic currents, does have some drawbacks including: decreased reliability due to high component count and stress on switching components Q1 and D1; a reduction in the overall efficiency of the supply due to a typical efficiency of the boost regulator of no better than 95%; and, increased high frequency conducted and radiated emissions back on to the AC mains due to the high switching frequency of the boost regulator, which imposes an additional cost and imposition of providing more input EMI (electromagnetic interference) filtering to meet emission standards.
An approach typically taken to provide a universal input power supply without power factor correction is shown in FIG. 3 which depicts a switching power supply 30 including an electronic switch (such as a triac device 35) gated by a controller/drive device 37 that electronically configures the triac 35 (T1) to be ON or OFF based upon capacitor voltages (C1, C2). Particularly, the controller/drive device 37 may comprise a universal voltage monitor, e.g., the ON Semiconductor™ device part number MC34161, MC3 3161 which is enable direct monitoring of positive and negative voltages. This approach has the benefits of requiring only a few components to achieve universal input, also, the efficiency of the supply remains unaffected at high line (>180 VAC input) since triac T1 is OFF for this line range. In addition, the overall reliability of the supply is better than in the active power factor correction (PFC) front end of FIG. 1, due to the circuit's low part content and low stress count. Furthermore, in the embodiment of FIG. 3, the electromagnetic interference (EMI) problem is reduced due to the static nature of this topology. However, the switching power supply circuit 30 will not provide harmonic current reduction, and for power levels above 200 W will not provide compliance to EN61000-3-2. This is because the circuit will generate the same AC line current waveform as in the straight rectified capacitor front end without a triac. A diagram showing the line current for this case is shown in FIG. 4(a) which depicts the AC line current 36 provided with the triac switch T1 turned on as compared with the a.c. line voltage Vac 39. In this configuration, the duration of the Iac current flow peak 38 is about 2 milliseconds, however, as known, is dependent upon the magnitude of capacitors C1 and C2 and the load thereon and is dependent upon a crest value of the ac waveform.
It would be highly desirable to provide a switching mode power supply circuit utilizing having a simple auto ranging front end such as described above with respect to FIG. 3 that complies with the EN61000-3-2 standard.
It would be highly desirable to provide a switching mode power supply circuit having a simple auto ranging front end such as described above with respect to FIG. 3 that complies with the EN61000-3-2 standard and includes the addition of a switched passive element for ensuring harmonic current limiting at certain line level voltages.